The present invention relates to a process and a device for the production of plastics, and particularly foams, from at least two free-flowing, metered reactants. At least one of the reactants is continuously pre-mixed with a pulverulent to fine-grained or fibrous additive in a prescribed ratio under pressure in a preliminary mixing zone. The premixture is subsequently brought together with the other reactant, and continuously mixed. The complete mixture is then allowed to react to completion.
More and more frequently, in order to obtain particular properties in the final product, pulverulent to fine-grained or fibrous additives have to be mixed with the starting components. For example, short glass fibers increase the strength properties. Low-cost fillers make the product less expensive. Pulverized foam scrap can be recycled and the addition of melamine and melamine resins increases the flame resistance.
Metering reactants and additives during continuous mixing at a constant ratio over time presents difficulties. This is due to the fact that pulverulent to fine-grained or fibrous products exhibit large fluctuations in bulk density. Blockages can easily result during metering, so that the throughput is irregular and the metering is inexact.
Substantially pressureless metering of pulverized foam scrap into the polyol component in a preliminary mixer during the production of polyurethane foams has already been attempted (see, e.g., German patent 2,540,934 corresponding to U.S. Pat. No. 4,068,831). A sheath is provided at the feed position. The sheath prevents the additive introduced from settling on the wall as a result of electrostatic charges. In this case exact metering was not so important, and the general metering difficulties with pulverulent products could not be eliminated with this device.
The object of the present invention was to provide a process and a device with which not only the metering ratio of the reactants, but also that of the additive are maintained exactly under continuous operation.